1. Field of the Invention
The invention relates to gasket materials used for attaching and sealing covers to enclosures. More particularly, the invention relates to form-in-place gaskets, applied to surfaces of containers for sensitive electronic components. Gasket compositions, according to the present invention, may be cured by a process including exposure to actinic radiation to convert them to a condition substantially free from deleterious effects of outgassing and ion contamination.
2. Discussion of the Related Art
Conventional methods for gasket manufacture substantially comprise either die-cutting the gasket out of an elastomeric sheet material, or shaping the gasket by injection-molding of an elastomeric mix or the like. Both these methods require expensive tools such as punches and molds, which add cost to the final product. Newer manufacturing methods deposit a bead or thread of a fluid elastomer from a nozzle onto a substantially planar surface. The pattern adopted by the fluid elastomer thread may be controlled using automated equipment, programmed according to selected coordinates to provide a gasket having a desired shape. After forming to a desired gasket pattern, the fluid elastomer thread may be cured, either at ambient temperature or in an oven, with or without accelerators or other additives.
Fluid elastomer compositions, suitable for form-in-place gaskets, include condensation-reaction curing silicone rubbers and addition-reaction curing silicone rubbers. These compositions have viscosities suitable for application using robot applicators which apply a sealing bead of material to at least one surface of at least one of the joining members. According to U.S. Pat. Nos. 4,643,863 and 4,643,864, fluid elastomers, suitable for controlled automated dispensing from a nozzle, include polyurethane, monocomponent or bicomponent silicone, and even polyvinylchloride compositions. An apparent disadvantage, of previously cited, nozzle-dispensed materials, is the need to provide support for the extruded bead of fluid elastomer. This problem was overcome with the development of a silicone rubber composition described in U.S. Pat. No. 5,684,110. Upon application of this silicone rubber composition to a substrate, it exhibits excellent resistance to distortion, under pressure, immediately after being applied and while curing to a highly pressure-resistant and strongly adhering silicone rubber gasket. In this case the silicone gasket composition cures via a combination of condensation-reaction curing and addition-reaction curing, the latter catalyzed with a platinum catalyst. The patent (U.S. Pat. No. 5,684,110) further reveals that the silicone rubber composition is a two-part formulation requiring a first silicone containing fluid to be added to a second fluid, comprising a silicone and catalyst, with intimate mixing immediately before robot application.
Another two-part silicone formulation, disclosed in U.S. Pat. No. 5,679,734, relates to compositions, which can be crosslinked by hydrosilylation at room temperature, in the presence of a metal compound catalyst. Crosslinking proceeds via an addition reaction involving hydrogen substituents and alkenyl radicals of the vinyl type. Storage stability is achieved by providing a system having at least two component parts. Upon mixing the component parts, a gel forms between a few minutes to 1 hour 30 minutes depending on the cure temperature which may be between room temperature and 180xc2x0 C.
Silicone materials, of the type previously discussed, meet the needs of a variety of applications including use as sealants, shock-absorbing elements, anti-vibration elements and gaskets in electrical and electronic components. One special and particularly interesting form of gasket is the electromagnetic interference (EMI) shielding gasket. EMI shielding gaskets perform the dual protective function of sealing cover assemblies and enclosures to prevent ingress of contaminants and, at the same time, exerting control over interference from electromagnetic energy. Protective sealants, effective in controlling EMI, may be used as gaskets that typically require a flexible, elastomeric, rubber-like matrix filled with a conductive material that is readily distributed throughout the flexible matrix. The conductive material may take the form of particles, flakes or fibers having intrinsic conductivity, or electrically conductive surface coatings. U.S. Pat. No. 5,641,438 discloses conductive sealant materials for application using form-in-place methods, which accomplish accurate positioning of the sealant bead. Similar compositions and methods are revealed in related published European applications, EP 0643551 and EP 0653552. Each reference describes EMI shielding sealant compositions, consisting of two or more components, requiring storage in separate containers and mixing just before applying and curing the gasket in place.
The previous discussion addressed primarily silicone-based fluid elastomer compositions suitable for use in a variety of applications including form-in-place gaskets. One disadvantage of using silicone elastomers is the presence of relatively low molecular weight siloxane contaminants in cured materials. Such contaminants tend to deposit on surfaces of an electronic assembly with the potential to cause device failure. Problems of contamination may be avoided using fluoroelastomer gaskets. Since they are subject to formation by injection molding, fluoroelastomer gaskets represent a costly approach for preventing contamination. Difficulties associated with non-fluid gaskets and fluid silicone elastomers, for gasket formation, suggests the need for a non-silicone, dispensable, fluid material for contaminant-free, low cost, sealing of containers of electronic components and associated devices.
The present invention provides fluid elastomer compositions having reactive functionality, preferably in the form of epoxy groups. Dispensable elastomer compositions provide dispensable, form-in-place gaskets designed for containers such as enclosures for hard disk drives. For accuracy of dimensions, placement and final positioning, various patterns of form-in-place gaskets, according to the present invention, require the dispensing of elastomer compositions using automated liquid dispensing followed by in-place curing to soft, resilient gaskets, exhibiting moisture resistance, minimal compression set, and adhesion to selected substrates. The elastomer composition, before curing, should have sufficiently low viscosity to be readily dispensed. Curable compositions may be prepared as one-part formulations that include a curative or as two-part formulations requiring addition of a curative before curing. Regardless of the components included in an uncured gasket composition, the curing process may be initiated thermally, photonically, a combination of both, and/or, in the case of two part formulations, by simply combining the parts under ambient conditions. One preferred embodiment uses photocuring to initially solidify a gasket bead, which is then heated to complete the cure and aid removal of remaining volatile components. Preferably uncured formulations provide non-slumping dispensed beads of gasket material having good dimensional stability, as applied, with essentially no change in shape or position during and after curing.
For electronics grade cleanliness, properties of these elastomer compositions, after curing, include low outgassing and low extractable ionics. These properties surpass those of commercially available, silicone-based form-in-place gaskets which, as previously discussed, typically contain low molecular weight siloxanes that can damage electronic components following contamination of device surfaces. Since this invention utilizes a flexible epoxy based material, possible damage due to siloxane contamination is avoided.
More particularly the present invention provides a non-silicone composition for form-in-place gaskets comprising a liquid polyolefin oligomer, a reactive diluent, a thixotropic filler, and a curative. The non-silicone composition, after curing, has a compression set of about 7% to about 20%, preferably about 10% to about 15%, a level of outgassing components of about 10 xcexcg/g to about 45 xcexcg/g and a Shore A hardness from about 45 to about 65 preferably from about 50 to about 60.
Non-silicone compositions according to the present invention may be dispensed using a variety of methods and equipment including commercially available fluid dispensing equipment. Gasket dispensing and placement may involve a variety of different methods. Speedline Technologies Inc. offers suitable commercial liquid dispensing equipment under the tradename CAMALOT, e.g. CAMALOT 1414 and CAMALOT 1818.
The present invention further includes a non-silicone, form-in-place gasket produced using automated placement, followed by curing, of a pattern of an extrudable thixotropic non-silicone composition. The extrudable composition comprises, a liquid polyolefin oligomer, a reactive diluent, a thixotropic filler, and a curative. The form-in-place gasket, after curing, has a compression set from about 7% to about 20%, preferably from about 10% to about 15%, a level of outgassing components from about 10 xcexcg/g to about 45 xcexcg/g, and a Shore A hardness between about 45 and about 65, preferably between about 50 and 60.
Photocurable compositions according to the present invention provide form-in-place gaskets using automated placement followed by photocuring of a pattern of a non-silicone composition comprising a liquid polyolefin oligomer, a reactive diluent, and a curative. The form-in-place gasket, after curing, has a level of total outgassing components of about 10 xcexcg/g to about 45 xcexcg/g. The curative included in form-in-place gaskets according to the present invention responds to actinic radiation and heat, and may contain a photoinitiator. Optionally a form-in-place gasket according to the present invention further comprises a thixotropic filleri in an amount from about 8.0 wt. % to about 12.0 wt. %, and preferably comprises a fumed silica.
A process for forming a cured form-in-place gasket, having a total level of outgassing components of about 10 xcexcg/g to about 45 xcexcg/g, comprises a number of steps after providing a photocurable, non-silicone composition comprising a liquid polyolefin oligomer, a reactive diluent and a curative responsive to actinic radiation and heat. After dispensing the photocurable composition as a form-in-place gasket bead it is exposed to actinic radiation, preferably ultraviolet radiation, to increase the rigidity of the gasket bead. Heating the gasket bead to an elevated temperature further cures the gasket bead and substantially displaces any volatile components therefrom to produce the finally cured form-in-place gasket.
The term xe2x80x9cdispensablexe2x80x9d means that low viscosity elastomer compositions may be conveniently extruded from tubes, such as needles, attached to pressurized reservoirs, to provide small diameter (xcx9c1 mm) beads of sealant following the contours of a required gasket pattern.
The combination of terms xe2x80x9cone-part, cure-in-placexe2x80x9d refers to fluid elastomer compositions containing a curative that responds to heat or actinic radiation to promote crosslinking and curing of the one-part formulation, once dispensed onto its substrate.
The term xe2x80x9cphotocuringxe2x80x9d refers to the use of actinic radiation preferably ultraviolet radiation, to produce reactive species that promote crosslinking and curing of monomers, particularly epoxy monomers present in form-in-place gaskets according to the present invention. Preferred photoinitiators include onium salt photoinitators.
The term xe2x80x9ccurativexe2x80x9d as used herein may include a xe2x80x9cphotoinitiatorxe2x80x9d for the purpose of photocuring as above.
The term xe2x80x9cnon-slumpingxe2x80x9d refers to the properties of an elastomer composition, including yield stress and viscosity that deter sagging or slumping during dispensing and curing. A non-slumping material is important for maintaining cross-sectional profile stability of a dispensed bead, of elastomer composition, before and during curing.
The term xe2x80x9caspect ratioxe2x80x9d indicates the slumping tendency or dimensional stability of a gasket composition according to the present invention by measuring the height and width of a cross-section through a cured, extruded bead of the composition. A cross sectional height divided by the corresponding width provides a value of xe2x80x9caspect ratioxe2x80x9d of a cured gasket bead.
The term xe2x80x9chydrophobicxe2x80x9d relates to the water repellency of elastomer compositions, which provide sealing gaskets with moisture barrier properties.
The term xe2x80x9celectronics-grade cleanlinessxe2x80x9d means that cured elastomeric compositions for form-in-place gaskets satisfy electronics industry requirements such as low levels of outgassing and extractable ionic contaminants. Experimental investigation by dynamic headspace gas chromatography (GC)/mass spectrometry shows a total outgassing of about 10 xcexcg/g to about 45 xcexcg/g for cured elastomer compositions, according to the present invention, when held at 85xc2x0 C. for three hours.
The term xe2x80x9cflowabilityxe2x80x9d of uncured gasket material, as used herein, refers to the amount of material passing through an opening of fixed diameter under a fixed pressure for a selected time interval. xe2x80x9cFlowabilityxe2x80x9d is expressed as the weight of material exiting through the orifice during the time that pressure was applied.
The term xe2x80x9coutgassingxe2x80x9d relates to the collection of volatile components produced by evacuation of cured non-silicone gasket compositions according to the present invention. Established criteria require microgram (xcexcg) quantities of volatile components for each gram (g) of composition evaluated.
Material amounts in compositions according to the present invention are given as percent by weight (wt %) unless otherwise indicated.